Despite persitent harassment and even imprisonment, Poland and Czechoslovakia are upholding their tradition as the East Bloc countries with the most active dissidents.

In Czechoslovakia, the human-rights movement has beefed itself up after its principal activists were jailed last fall. It has enlarged its committee in Prague and added members from four provincial centers.

Three of its spokesmen behind bars -- dramatist Vaclav Havel, journalist Jiri Dienstbier, and scientist Vaclav Benda -- managed to get an appeal smuggled from their Moravian jail to the Paris meeting April 28 and 29 of European communist parties. But the meeting was attended only by pro-Soviet parties, and their message fell on deal ears.

And in Poland, five prominent writers have made a public protest over the March arrest of Miroslaw Chojecki that, in effect, challenges both the state's censorship and the regime's exclusive control of the publishing and printing industry.

Their protest won't result in reversal of the arrest. But their ability to make such a point without official reprisal reflects a degree of tolerance not shown in Prague. There the regime recently rid itself of several troublesome writers by allowing them to "visit" the West and then blocking their return by annulling their citizenship.

At the time of his arrest, Mr. Chojecki was running an unlicensed publishing collective that had put out several books viewed unfavorably by the authorities. He is charged with involvement in the theft of "government property" -- a duplicating machine removed from a state print shop that was apparently used in producing unauthorized material.

The Polish police apparently have had some success in disrupting the "flying university," the school through which academics denied teaching positions hold unofficial classes in private homes for students denied access to state universities. But they have been less successful in quashing publishing efforts like Mr. Chojecki's.

Czechoslovak authorities have similarly been trying to squelch quasi-underground "university" classes held by academics dismissed after the collapse of the Dubcek regime in 1968.